In recent years, the need for optical elements employing high-refractive-index, high-dispersion glass has increased immensely. Such glass optical elements are extremely important optical components in the optical systems of cameras and digital cameras, in particular. Further, methods of molding aspherical lenses and the like by precision pressing of high-refractive-index, high-dispersion glass without grinding or polishing are required. For example, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 7-10556 describes temperature ranges for a pressing mold and glass material that are suited to precision pressing.
Examples of known high-refractive-index, high-dispersion glass are described in Japanese Unexamined Patent Publications (KOKAI) Heisei No. 1-308843 and Showa No. 62-3103. However, these glasses contain a large amount of PbO. Thus, they are undesirable from an environmental perspective, and present a further problem in that lead is reduced in the molding chamber during the press molding of optical components, precipitating out into the pressing mold. Due to the relatively high sag point of these glass gobs, the molding temperature must be raised. Thus, there is a problem of deterioration of the mold material during molding and a problem of crystals precipitating in the glass during molding.
Further, Japanese Unexamined Patent (KOKAI) Publication No. 2001-58845 discloses a high-refractive-index, high-dispersion glass with a refractive index (nd) equal to or greater than 1.83 and an Abbé number (νd) of less than or equal to 26. However, when this glass is heated to a temperature suited to press molding, components tending to reduce (Nb2O5, WO3, TiO2) react at the mold surface, causing problems by damaging the release film applied on the mold surface, generating bubbles on the surface of the glass material, and producing radial line marks on the surface of the element following pressing. Accordingly, the satisfactory quality of lens is not necessarily obtained.
Accordingly, the object of the present invention is to provide a method of manufacturing glass optical elements by press molding high-refractive-index, high-dispersion glass without damaging the release film applied on the mold surface, without generating bubbles on the glass material surface, and without generating radial line marks on the optical element surface, while preventing cracking during molding.